This invention relates to a control system, such as a spacecraft attitude control system, having a choice of gains to be employed in a controller and in a compensator of the system and, more particularly, to a mode of switching between a high gain and a low gain without introduction of a significant transient.
Control systems attempt to control a plant, such as a spacecraft, submarine, chemical plant, or manufacturing equipment, by way of example, by outputting desired quantities, such as motor torques and thruster assignments with on times, in response to the sum of these quantities and disturbances on the plant. Herein, the plant of primary interest is a spacecraft and, accordingly, this presentation will be in terms of a spacecraft, it being understood that the concepts presented herein apply also to other plants. For example, in the case of a spacecraft employing an earth sensor to determine spacecraft attitude by observing the edges of the earth to provide an estimate of nadir, it is a practice to use thrusters and reaction wheels to reorient a spacecraft to maintain its attitude. Typically, a control system is configured as a feedback loop wherein a desired output is the spacecraft orientation and its rates. Typically, an estimator is employed to provide estimates of these quantities, these quantities being called the states of the spacecraft. An estimator is not required if direct, low noise measurements are available, or can be transformed into the direction of these states. Earth sensors and rate gyros are examples of sensors that can be used for direct measurements of the states of spacecraft orientation and rates. However, most spacecraft require pointing to higher accuracies than the earth sensor noise levels, thereby requiring some sort of filtering or estimation for the desired results. The difference between the desired set of states and the estimated (measured) set of states is known as the loop error signal.
Information obtained from the earth sensor, as well as from another type of sensor such as a star sensor, or an image recognition device for recognizing features of the earth, by way of example, often is applied to an estimator to filter out noise of the measurement to gain a higher precision of the measurement. The estimator may process the signal outputted by one of the foregoing sensors, or may process signals outputted by a plurality of such sensors to provide estimates of the states of the plant being controlled. These estimates, when differenced with a set of desired states, provide the control system with a set of feedback signals. Estimating may involve averaging, statistical analysis and/or Kalman filtering, by way of example. The estimator may be provided with a relatively high gain for faster response with increased noise power output, or may be provided with a lower gain for slower response. The lower gain results in a slower response and lower bandwidth providing state estimates with reduced noise. Similarly, a controller of an output quantity, such as a rotational rate of the spacecraft, may be provided with a higher gain for faster response with increased noise power output, or may be provided with a lower gain for slower response and reduced noise power output. In the case of a reaction wheel controller, the higher gain may be used during a thruster firing, while lower gains may be used during quiescent periods.
The disturbance function may be in the form of a disturbing torque produced, by way of example, by viscous friction of a lubricant on a reaction wheel or by the pressure of solar rays on a spacecraft. The disturbing torques can have a cumulative effect which require constant torque demands from the control system even after the loop error signal is driven to zero. In order to reduce hang off errors resulting from these demands, some control system designs employ integral control states to generate a non-zero controller input signal in order to maintain these constant torque demands from the control system.
Control systems can be implemented with analog or digital hardware. In the case of an analog implementation, estimates of the plant states are generated with RLC (resistive, inductive, capacitive) circuits. These circuits filter the sensor outputs, which typically would be voltage levels, to the desired voltage levels for the control system to utilize. These output voltages are differenced with desired voltage levels to provide error signal inputs to the control system to use additional RLC circuits to generate output voltages to be sent to the actuators. In the case of a digital implementation, sensor outputs would be converted to binary data for the onboard computer to utilize. These data would then be manipulated with algorithmic processing to provide estimates of the plant states. These estimates are differenced with desired states to provide error signal inputs to the control system to use additional algorithmic processing to generate outputs to be sent to the actuators. In either case, one of the estimates could be the plant state corresponding to the orientation of the spacecraft, another estimate could be the integral of the difference between this estimate and the desired orientation of the spacecraft, another could be the integral of the difference between this state and the desired position of the spacecraft, or the raw integral of a sensor that measures that difference, a third state could be an estimate of the rate of spacecraft. Differences between these estimates, whether voltage levels or binary data, and the desired states provide a composite error signal to generate outputs to control actuators for improved response of the plant.
One aspect of a control situation, particularly in the case of the orienting of a spacecraft, is the need to vary a gain of the control system. For example, a thruster would be employed to change the orientation of the spacecraft. In the case of the orienting of the spacecraft to correct a drifting away from a desired orientation, use may be made of reaction wheels. Maintaining a torque from the reaction wheels to the spacecraft to reject disturbance torques can cause the wheel speeds to increase beyond their safe operating speeds. A desired form of response of the momentum control system would be to activate a thruster to reduce the momentum of the spacecraft. This impulse would temporarily shift the orientation of the spacecraft. The wheel control system would respond by reducing the wheel speeds, thereby restoring the spacecraft's desired orientation. These examples are given in terms of a single axis of rotation, but it should be understood that, in the typical situation, orientation is to be adjusted about three axes of rotation employing a plurality of thrusters and a plurality of reaction wheels.
Furthermore, in the foregoing example, it may be desired to increase the gain of the compensator of the sensor signals to give a faster response to position signals obtained from one or more sensors. A slow response in the compensator would inhibit observation of a rapidly changing situation, such as a rapid change in spacecraft orientation. This would negate control of the spacecraft orientation in a situation of rapid change of orientation. Accordingly, control systems typically increase their gains when the plant being controlled enters a condition of increased disturbance, such as the reaction wheel control system responding to thruster firings.
A problem arises in that, upon a changing of gain in a control system, such as from a high gain to a low gain or from a low gain to a high gain, it has been found that an undesired transient develops in the plant response. This might be manifested by excessive overshoot in the spacecraft position, an inadequate speed of response, or inadequate precision due to noise outputted by the estimator.